1. Field of the Invention
The present invention relates to a printing apparatus which prints image information on a recording medium such as a recording sheet.
2. Description of the Related Art
Printing apparatuses such as a computer output apparatus and a digital video output apparatus can be classified into a thermal transfer type printing apparatus, an ink jet printing apparatus, a laser printing apparatus, or a wire dot printing apparatus in accordance with a recording system of the apparatus. Among them, a linear thermal transfer type printing apparatus, as an example of the thermal transfer type printing apparatus, conveys an ink sheet and a recording sheet in a sub-scanning direction while selectively driving a plurality of heat generators arranged in a main scanning direction. In consequence, dot lines are printed on the recording sheet.
In another type of printing apparatus such as an ink jet printing apparatus, there is only binary selection of whether or not to form dots. Therefore, while small dots are formed on the recording sheet, apparent resolution and gradation are to be obtained by a technique such as error diffusion. On the other hand, in the thermal transfer type printing apparatus, a thermal value can easily be changed so that one pixel can be controlled. Therefore, a lot of gradations can be taken with respect to one pixel. As a result, a smooth image having a high quality can be obtained as compared with another printing apparatus such as the ink jet printing apparatus.
FIGS. 6A and 6B are schematic diagrams showing a basic constitution of a conventional thermal transfer type printer. As shown in FIG. 6A, while an ink sheet 103 is brought into close contact with a recording sheet P by a thermal head 101 and a platen roller 102, the recording sheet P is conveyed downstream (in an arrow direction in the drawing) in a printing direction by a driving roller 104 and a driven roller 105.
At this time, the thermal head 101 is allowed to generate heat, thereby thermally transferring ink of the ink sheet 103 onto the recording sheet P to perform printing of a first color.
Subsequently, to perform the printing of the next color, at least one of the thermal head 101 and the platen roller 102 is moved so that they come away from each other. Next, the driving roller 104 and the driven roller 105 are rotated in a direction reverse to that of the first-color printing to return the recording sheet P to a printing start position, and the second-color printing is performed in the same manner as in the first-color printing. Subsequently, the above operation is repeated to perform printing of third and subsequent colors.
As a conventional driving mechanism which causes the thermal head and the platen roller to come close to each other or come away from each other, an example to adjust a position of the thermal head with respect to a platen is disclosed in Japanese Patent Application Laid-Open No. H06-015852.
Moreover, an example to adjust a position of the platen with respect to the thermal head is disclosed in Japanese Patent Application Laid-Open No. H10-076716.
FIGS. 7, 8A and 8B are diagrams showing a driving mechanism which causes the platen roller to come close to the thermal head or to come away from the same in the conventional thermal transfer type printer. FIG. 7 is a perspective view, FIG. 8A is a top plan view, and FIG. 8B is a side view. In FIGS. 7, 8A and 8B, a pair of left and right lever members 106a, 106b holds the platen roller 102. The members are capable of switching to a state in which the roller 102 comes close to the thermal head 101 or a state in which the roller comes away from the thermal head 101. A communication shaft 109 fixes a pair of left and right cams 107, 108 to the same shaft, and a gear 110 is fixed to one end of the communication shaft 109. A driving mechanism 111 rotates the gear 110. This driving mechanism 111 is constituted of a motor 111a, a worm gear 111b, a worm wheel 111c and the like.
Reference numerals 112a, 112b are rotary shafts of the lever members and the cams and worm wheel of the driving mechanism.
When the driving mechanism 111 rotates the cams 107, 108 at a predetermined angle via the gear 110 and the communication shaft 109, the lever members 106a, 106b cause the platen roller 102 to come close to the thermal head 101, or to come away from the same. FIG. 9 shows a state in which the cams 107, 108 rotate at the predetermined angle, so that the platen roller 102 comes away from the thermal head 101.
However, the conventional driving mechanism which causes the thermal head and the platen roller to come close to each other or to come away from each other as described above has to be provided with a pair of cams on opposite outer sides of a region where a recording medium is conveyed. Therefore, there has been a problem that miniaturization of the printing apparatus is restricted.
As seen from FIG. 8B, the ink sheet 103 is disposed in the vicinity of the thermal head 101. In this ink sheet 103, as shown in FIG. 10, an ink sheet portion 121 which comes into contact with the thermal head is sandwiched between ink sheet portions 103 and 120 which are to be fed out or taken up and which are wrapped in an outer package and formed into a cartridge. A constitution is general in which the cartridge is detachably attached in a longitudinal direction of the thermal head 101, that is, in an arrow B direction of FIG. 8A. Therefore, as viewed from a side surface in FIG. 8B, the cartridge cannot be laid out in a region on which the pair of cams 107, 108 (a region E in FIG. 8B) arranged on the opposite sides are superimposed.
Here, when the cams 107, 108 can be designed to be large, a large reduction ratio or a large stroke can be achieved. Therefore, the cams can be designed so as to be as large as possible.
Moreover, a diameter of a sheet roll of the rolled ink sheet 103 can be designed to be large. This is because the large roll diameter can lengthen a replacement cycle of the roll. There are advantages that a take-up radius is increased to set a take-up torque to be constant and that a tensile force of the ink sheet 103 to be fed out or taken up can relatively be reduced.
Furthermore, considering the miniaturization of the apparatus, it is important that components such as the cams 107, 108 and the ink sheet 103, which are a part of the driving mechanism for causing the thermal head 101 and the platen roller 102 to come close to each other or to come away from each other, should be arranged in the vicinity of the thermal head 101.
As described above, the cams and the ink sheet can be designed so as to be large in order to further improve a function of feeding out or taking up the ink sheet, a function of adjusting a distance between the platen and the thermal head and the like. Moreover, there is such a restriction that the components cannot be laid out in a region where they are superimposed on each other, although they should be arranged in the vicinity of the thermal head. This is a large design problem. For such a reason, there is a restriction on the layout of the cams and the ink sheet, and this is a problem in miniaturizing the printing apparatus.